The sheets of silicon steel with oriented magnetic grain find wide use in the production of cores for transformers; in this use, one of the characteristics of the material subjected to deep analysis relates to the energy dissipated on the operation of the transformer. The losses depend on several factors and, in particular, the movement of the walls of the magnetic domains, the grain sizes of the sheet and the direction of the crystalline reticulum of the grain with respect to the surface of the sheet.
A first solution is to provide sheets of silicon steel with high oriented grain, having given dimensions of the grain and low thickness: The obtained results are good, but cannot be substantially improved and in industrially acceptable way. In particular it has been found that the optimal dimension of grains is of some millimeter while, with respect to the thickness of the sheet, it is not convenient to go below determined values, in view of the cost of the treatments, and the fact that it reduces the “spatial factor” between volume of the sheet and volume of the insulating coverings.
An important factor that affects the losses in the cores relates to the dimensions of the magnetic domains. Applying a mechanical tension to the sheet, an anisotropy in the plan of the sheet is induced, which, in presence of the typical structure of the used material (texture of Goss), increases the energetic difference of magnetization between the crystallographic direction, parallel to the direction of lamination, and the direction perpendicular to the direction of lamination. The balance between magneto-static energy and energy of the walls of the dominions is in favor of the energy of the walls, causing the formation of a greater number of walls that become finer and closer. In such a way, a remarkable reducing of the contribution of the eddy currents to the total amount of the losses is obtained. Further, tensioning coverings have been developed, in order to obtain these improvements by creating localized compressive micro stresses. In this framework, it has been proposed to subject the sheet to shot blasting or to mechanical threading. Such methods can be applied with difficulty in the industry, destroy the insulating covering with exposure of the sheet to rapid oxidation, require further covering, and form flashes, whereby diminishing the spatial factor.
A successive solution, largely consolidated, has been to subject the surface of the sheet, as electromagnetic steel strip with oriented grains unwinding from coils, to scratching or threading (scribing) with energetic impulses concentrated in form of laser beams, plasma and similar.
In typical equipments for the treatment of scribing directed to the improvement of electromagnetic characteristics, the beam of a laser generator is deviated on the moving strip by a mirror scanner and then focused on the sheet along a scanning path transversal to the direction of advancing of the strip.
In order to obtain an appreciable reduction of the losses, the treatment of scribing should be of reduced cross-sectional dimensions, as an example 0.10 mm and extended at a maximum in depth. Good results have been obtained by using laser beams with spot of hardly elliptic section, lengthened in the sense transversal to the direction of advancing of the strip. It has been performed by means of suitable cylindrical optics.